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ABSTRACT

Midcrustal channel flow has been hypothesized to be responsible both for the Greater and Lesser Himalayan Sequences (the Miocene Himalayan channel theory), and for the present eastward and northward movement and extension of the Tibetan upper crust (the Tibetan middle-crustal channel flow theory). Because processes within the crust cannot be directly observed, various studies have attempted to validate midcrustal channel flow theory by using indirect approaches, including outcrop patterns and other field data from the Himalayas, Tibet, and exposed older orogenic roots. The results have been highly debated, because arguments can be made that the internal structure of a channel and, therefore, the outcrop patterns of a paleomidcrustal channel are not unique. This paper investigates the types of structural patterns that may be produced within a midcrustal channel and discusses why they can be difficult, if not impossible, to distinguish from outcrop patterns produced by other mechanisms. A new example from the exposed middle crust of southern Finland is also discussed in this context. While outcrop structural patterns must indeed agree with other potential results that may imply a midcrustal channel, the inverse is not necessarily true: One cannot infer a midcrustal channel based on outcrop patterns alone, due to the nonunique nature of the patterns.

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